Metal injection molding method

ABSTRACT

The present invention discloses a metal injection molding method, which is adapted to a mold having multiple mold cavities, and includes a feedstock preparation step, a molded articles ejection step, a classification and management step, a wax-based material removal step, a sintering step, and a compacting step. The classification and management step is to classify molded articles according to differences of the molded articles. The sintering step is to sinter the molded articles with sintering parameters. In this manner, tolerance caused by injection molding process is reduced because of the classification and management step and the sintering step, whereby further improving productive rate and quality of products produced from the mold with multiple mold cavities.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an injection molding method, and particularly to a metal injection molding method adapted to a mold with multiple cavities for reducing tolerance and producing high precision products.

2. Related Art

In brief, a process of metal powder injection molding (MIM) includes steps of preparing a feedstock for being fed into a plastic molding machine to be shaped, then cleaning with solvents or heating the shaped article to remove wax-based materials, then sintering the shaped article in a sintering furnace to remove all the other binders, and compacting the metal powder at last.

Electronic products today have extremely small parts inside and are fabricated in relatively high precision. Thus dimensional tolerance is required to be as small as a nearly zero dimensional difference among products. Traditionally, an injection mold is provided with one mold cavity in order to effectively control the product dimension. However, it is not preferable to mass produce through one mold with one cavity because of a lower production rate. As a result, one mold with multiple cavities has been designed to produce multiple same products at a time by one injection molding. Although one mold with multiple cavities can increase the production volume, it is also affected by more variables during the molding process. For instance, the gravity acceleration probably will cause the feedstock material to flow in different speeds into different mold cavities, and whether surfaces of flowing channels are smooth or tough influences material flow property as well, whereby leading to a visible dimensional tolerance between products.

Additionally, referring to FIG. 1, a traditional sintering furnace 9 has a chamber 92 typically surrounded by a plurality of heating elements 91. Numerous articles 93 are put in the chamber 92 to undergo a process of sintering including heating, holding time, cooling and other steps so as to produce metal components. However, articles 93 molded from a traditional mold with multiple cavities are placed disorderly in the chamber 92 in a manner such that articles from a same cavity are spread randomly in the chamber 92 (as shown in FIG. 1). As a result, articles from the same cavity are kept different distance from the heating elements 91 to each other. However, distance between the heating elements 91 and articles is known to significantly influence heating efficiency no matter it is heated through thermal radiation, thermal convection, thermal conduction or other methods. Different distance between articles and heating elements, different heating efficiency, and dimensional tolerance all result in remarkable dimensional differences of end products which are certainly not qualified to meet requirements.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide a metal injection molding method adapted to a mold with multiple mold cavities, utilizing classification of molded articles and setting of sintering parameters to improve productive rate and produce high precision products.

To achieve the above-mentioned objects, the metal injection molding method, adapted to a mold with multiple mold cavities, comprises: a feedstock preparation step for mixing metal powder and a binder to produce a feedstock; a molded articles ejection step for injecting the feedstock into the multiple cavities of the mold so as to shape the feedstock as multiple molded articles, and then ejecting the molded articles from the mold; a classification and management step for individually managing and classifying the multiple molded articles according to differences among the multiple molded articles; a wax-based material removal step for cleaning the molded articles with solvents or heating the molded articles to remove the binder; a sintering step for sintering the multiple molded articles with sintering parameters where the multiple molded articles are placed in a sintering chamber according to classifications of the multiple molded articles; and a compacting step for compacting the multiple molded articles after being sintered so as to produce end products.

With the above-mentioned method, tolerance caused by injection molding process is reduced because of the classification and management step and the sintering step, whereby further improving productive rate and quality of products produced from the mold with multiple mold cavities.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cutaway view showing a sintering chamber stored with multiple articles of prior art;

FIG. 2 is a schematic top plan view of a mold with multiple mold cavities of a metal injection molding method of the present invention;

FIG. 3 is a detailed flowchart of the metal injection molding method of the present invention; and

FIG. 4 is a schematic cutaway view showing a sintering chamber stored with multiple articles according to classifications of the multiple articles of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 3 illustrating a detailed flowchart of a metal injection molding method 100 in accordance with the present invention, the process 100 is adapted to a mold 7 with multiple mold cavities 70 (as shown in FIG. 2) and to produce high precision metal parts. In this preferable embodiment, the mold 7 has four corresponding mold cavities 70. The metal injection molding method 100 comprises a feedstock preparation step 1, a molded articles ejection step 2, a classification and management step 3, a wax-based material removal step 4, a sintering step 5, and a compacting step 6.

The feedstock preparation step 1 is intended to prepare a feed stock by mixing metal powder and a binder.

The molded articles ejection step 2 is intended to produce multiple molded articles 71, 72, 73, 74 (also referred to green part) by injecting the feedstock into the multiple cavities 70 of the mold 7 so as to shape the feedstock, and then eject the shaped feedstock as the molded articles 71, 72, 73, 74 from the mold 7.

The classification and management step 3 is intended to individually manage the multiple molded articles 71, 72, 73, 74 so as to classify them according to differences among the multiple molded articles 71, 72, 73, 74.

The wax-based material removal step 4 is intended to remove the binder by cleaning the molded articles 71, 72, 73, 74 with solvents or heating the molded articles 71, 72, 73, 74.

The sintering step 5 is intended to form sintered articles (also referred to brown part) by sintering the multiple molded articles 71, 72, 73, 74 with sintering parameters where the multiple molded articles 71, 72, 73, 74 are placed in a sintering chamber 81 according to classifications of the molded articles 71, 72, 73, 74.

The compacting step 6 is intended to compact the sintered articles 71, 72, 73, 74 so as to produce end products.

Particularly, the differences among the multiple molded articles 71, 72, 73, 74 in the classification and management step 3 include dimensional difference, surface difference, different mold cavities 70 or any other differences. Such differences are capable of being distinguished by eyes or hands or instruments. Classifications of the molded articles 71, 72, 73, 74 are determined according to such differences. For example, when classified according to the four mold cavities 70, the molded articles 71, 72, 73, 74 are classified into four classifications, each one of the mold cavities 70 representing one classification. The multiple molded articles 71, 72, 73, 74 are placed in the sintering chamber 81 respectively according to their classifications so as to be sintered with the sintering parameters at high temperature.

The sintering parameters are set to control a temperature, time, pressure, and atmosphere during sintering, and may be varied according to classifications of molded articles. One of the embodiments of sintering is that molded articles from the same mold cavity are being sintered together at a time, while other molded articles from other mold cavities are being sintered separately with different parameters. Another embodiment of sintering is that different classifications of molded articles are being sintered with different parameters. Alternatively, different classifications of molded articles are capable of being sintered with same parameters, wherein further changes of the molded articles resulted from the process of sintering are capable of counterbalancing tolerance resulted from the process of injection molding. Furthermore, a secondary processing step is able to be performed after the compacting step 6, such as electroplating, polishing, or sandblasting.

Referring to FIG. 4 illustrating an embodiment of the molded articles 71, 72, 73, 74 that are placed in the sintering chamber 81, in the classification and management step 3 the molded articles 71, 72, 73, 74 are classified according to their dimensional difference. In this embodiment, the molded articles 71 of larger size are placed at peripheral portions of the sintering chamber 81 in order to be close to the heating elements 82 to improve a heating efficiency. The molded articles 74 of smaller size are placed at a middle portion of the sintering chamber 81 where the heating elements 82 are further away from the molded articles 74 causing a lower heating efficiency. As a result, during the process of sintering, tolerance, caused by injection molding, of both the molded articles 71 of larger size and the molded articles 74 of smaller size is reduced because of different heating efficiency. In other words, after being sintered the molded articles 71 of larger size will become smaller whereby to ensure that each molded article from the same mold has almost the same dimension and to achieve the purpose of mass-produced by one mold with multiple mold cavities.

Accordingly, the present invention utilizes the classification and management step 3 to preliminarily classify the green parts so as to place the green parts appropriately in the sintering chamber 81 according to different classifications. Furthermore, the sintering step 5 is performed to set the sintering parameters in order to reduce tolerance caused by injection molding to improve the productive rate and yield rate of the metal injection molding method adapted to one mold with multiple mold cavities, whereby achieving the purpose of mass production of high precision products and greatly reducing the number of unqualified products, so as to effectively control the cost of manufacturing.

It is understood that the invention may be embodied in other forms within the scope of the claims. Thus the present examples and embodiments are to be considered in all respects as illustrative, and not restrictive, of the invention defined by the claims. 

What is claimed is:
 1. A metal injection molding method, adapted to a mold with multiple mold cavities, comprising: a feedstock preparation step for mixing metal powder and a binder to produce a feedstock; a molded articles ejection step for injecting the feedstock into the multiple cavities of the mold so as to shape the feedstock as multiple molded articles, and then ejecting the molded articles from the mold; a classification and management step for individually managing and classifying the multiple molded articles according to differences among the multiple molded articles; a wax-based material removal step for cleaning the molded articles with solvents or heating the molded articles to remove the binder; a sintering step for sintering the multiple molded articles with sintering parameters where the multiple molded articles are placed in a sintering chamber according to classifications of the multiple molded articles; and a compacting step for compacting the multiple molded articles after being sintered so as to produce end products.
 2. The metal injection molding method of claim 1, wherein the differences among the multiple molded articles in the classification and management step include dimensional difference, surface difference, or different mold cavities.
 3. The metal injection molding method of claim 1, wherein in the classification and management step the multiple molded articles are classified according to each of the mold cavities.
 4. The metal injection molding method of claim 1, wherein in the sintering step the molded articles are being sintered with different sintering parameters according to different classifications of the multiple molded articles.
 5. The metal injection molding method of claim 1, wherein in the sintering step the molded articles are being sintered with the same sintering parameters.
 6. The metal injection molding method of claim 1, wherein the sintering parameters are set to control a temperature, time, pressure, and atmosphere during sintering.
 7. The metal injection molding method of claim 1, wherein after being classified the molded articles of larger size are placed at peripheral portions of the sintering chamber, and the molded articles of smaller size are placed at a middle portion of the sintering chamber. 